1. Field of the Invention
The present invention relates to voltage-controlled oscillators, and more particularly to calibration circuits for VCO""s that include replica charge pumps.
2. Description of Related Art
Voltage-controlled oscillators (VCO""s) typically depend on a semiconductor device that varies its capacitance in relation to an applied voltage. Reverse-biased diodes can be fabricated to function as such, and are called varactors.
Conventional phase-locked loops (PLL""s) are used in prior art circuits to synthesize local oscillator frequencies used in radio receivers. The most common type of PLL uses a VCO that depends on a varactor. But inexpensive varactor diodes cannot be used in PLL""s that need to be tuned over very large frequency ranges, e.g., the VCO constant grows too large or the capacitance variation required becomes unachievable and the PLL has trouble locking. Such circuits when they do lock are very sensitive to digital noise, because a relatively small noise voltage is translated into a relatively large frequency perturbation. For example, in a 1.8-volt system needing a one gigaHertz tuning range, a VCO constant of 1 G Hz/volt would be far too large to be practical.
In general, a smaller VCO constant will result in reduced phase noise. It is very difficult to make good quality VCO""s in CMOS; the inductor Q""s, for example, are very poor. Therefore it is important that all other factors contributing to phase noise are minimized, the VCO constant being one of them.
The prior art has adopted the practice of switching in and out fixed capacitors and using the varactors to tune between. A one gigaHertz tuning range, for example, would be implemented in ten 100 MHz subranges, with the VCO constant of 100 MHz/volt, a twenty dB reduction.
Unfortunately, with mass produced semiconductor devices such switched fixed capacitors vary with manufacturing process spread and with operating temperatures. So a calibration method and circuit are needed that can reduce the frequency uncertainties that would otherwise be introduced into PLL and VCO applications. Better yet, a built-in calibration method could help in obtaining a longer, more reliable product life.
An object of the present invention is to provide a voltage controlled oscillator with a relatively large frequency swing with an output in the gigaHertz range.
Another object of the present invention is to provide a calibration method for a VCO in a mass-produced semiconductor device.
A further object of the present invention is to provide a wireless radio oscillator circuit.
Briefly, a local oscillator calibrator embodiment of the present invention comprises a main charge pump that drives a voltage-controlled oscillator (VCO) through a loop filter. A second, replica charge pump can also drive the VCO, but is setup to output only its most positive or most negative analog output control voltage. Since the construction and characteristics of the replica charge pump duplicate the main charge pump, the main charge pump""s minimum and maximum analog control outputs can be cloned out to the VCO on demand. A VCO calibration procedure therefore includes switching the VCO to each of its ranges set by a bank of fixed capacitors, and using the replica charge pump to drive the VCO to its minimum and maximum frequency for each range setting. The min-max frequency data is stored in a lookup table, and operational requests to switch to a new channel frequency can be supported with a priori information about which fixed-capacitor range selection will be best.
The above and still further objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed description of specific embodiments thereof, especially when taken in conjunction with the accompanying drawings.